Now you can apply one or more tags to tasks. You can create tags either in the Tag applet under the Burndown/Progress/Prediction or at the Manage Tags page. To apply a tag select one or multiple tasks using the checkboxes, then use Apply Tag action available in More Actions listbox. To filter tasklist by a tag click on tag's name in the applet. To undo the filtering click on All Tasks.
More: If you have ended up using "[Feature] Task Description" naming convention as an ad-hoc way of tagging tasks you can automatically tag this tasks. Just create "Feature" tag and use "Convert Brackets" button from Manage Tags page.
Even More: If a tag already exists and you create a new task with [TagName] prefix, the task will be automatically labeled for you.